Gas lubricated mechanical face seals, such as those described in U.S. Pat. No. 5,222,743 and U.S. Pat. No. 5,496,047 (the disclosures of which are incorporated by reference herein) rely on a cushion of seal gas built up between the sealing faces in order to provide lubrication, and thereby inhibit contact between the faces. In many applications, this lubricating gas is the process or product gas that is being compressed or processed by the mechanical energy transmitted by the spinning shaft. Depending on the application, such seals can be designed to operate at pressures up to 500 bar and speeds up to 30,000 rpm.
Non-contacting, dry-running mechanical seals generally include one or more seal interfaces defined by a mating (rotating) ring, fixable to a shaft, and a primary (stationary) ring, fixable to a housing. A seal interface is maintained, in part, by preloaded springs (or other biasing mechanism) arranged to urge the primary ring towards the mating ring. The springs often have sufficient force to push the primary ring into actual contact with the mating ring when the shaft is not rotating, or is rotating slowly (i.e., in standstill or slow roll conditions, respectively). When the rotating speed of the shaft is sufficiently high, grooves provided in the mating ring create sufficient fluid dynamic force to separate the primary ring from the mating ring, creating a gap between the two rings through which the lubricating gas can flow. The threshold (or lift-off) speed is dependent on various aspects of the seal design, however such thresholds are generally around 200 rpm.
In slow roll conditions, the fluid dynamic force is insufficient to overcome the spring force provided by the biasing mechanism to separate the primary and mating rings, which will then frictionally rotate against each other. This rubbing can lead to excessive heat generation and premature seal wear. Conventional approaches to limiting wear in slow roll conditions have included the application of a diamond-like carbon (DLC) coating to the seal faces. The DLC coating however requires an additional manufacturing step, and may not be sufficiently durable for all applications.
Another area of concern for mechanical seals is hang-up. Hang-up occurs when the primary ring becomes stuck in an axial position, wherein the spring preload force behind the carrier is insufficient to overcome a very high friction between the carrier and a sealing element arranged at the balance diameter. Hang-up is especially problematic where a seal system does not include an efficient gas conditioning unit, such that dirty hydrocarbon gas can migrate to the gas seal, where it can condense and build up on the sliding surface of the carrier in the balance diameter sealing element area, inhibiting the free movement of the carrier and primary ring. This can cause the carrier and primary ring to stick (or “hang-up”), either in the steady state position (resulting in a large seal face gap at start-up and the resultant excessive leakage as the casing pressurizes), or in the stand-still position (resulting in face contact and unacceptable face wear).